1. Field of the Invention
The present invention relates to a vibrating beam accelerometer and, more particularly, to a vibrating beam accelerometer, adapted to be excited electrostatically, which obviates the need for a relatively expensive housing to maintain a nearly perfect vacuum relative to the vibrating beams.
2. Description of the Prior Art
Various vibrating beam accelerometers are known in the art. Examples of such accelerometers are disclosed in U.S. Pat. Nos. 4,872,343; 4,901,586 and 5,005,413, all assigned to the same assignee as the assignee of the present invention and herein incorporated by reference. Such vibrating beam accelerometers are responsive to acceleration forces in an axial direction with respect to the beams which causes either a tension or compression force on the beams which, in turn, causes a variation of the frequency of vibration of the beams. This variation in the vibration frequency of the beams, in turn, is used as a measure of the acceleration.
Normally, such vibrating beam accelerometers are externally excited to cause the beams to vibrate at a resonant frequency. The type of excitation depends on the particular material from which the accelerometer is formed. Vibrating beam accelerometers are known to be formed from crystalline quartz and silicon. The vibrating beam accelerometers formed from crystalline quartz are excited according to the piezoelectric properties of the quartz. In particular, in such embodiments, electrode patterns are deposited on the vibrating beams and excited with an external source of electrical power. An example of a piezoelectrically excited vibrating beam accelerometer is disclosed in co-pending U.S. patent application Ser. No. 07/978,264, filed on Nov. 18, 1992 by Brian L. Norling and entitled "FOUR BARRESONATING FORCE TRANSDUCER", issued Nov. 22, 1994 as U.S. Pat. No. 5,367,217, herein incorporated by reference.
Vibrating beam accelerometers made from non-piezoelectric elements, such as silicon, are known to be excited either electrostatically or electromagnetically. However, there are various known problems with such accelerometers. For example, electromagnetically driven accelerometers require special electrical circuitry to enable the accelerometer to provide acceptable output signals over a relatively wide range of component manufacturing tolerances. Such circuitry requires a relatively large housing which increases the cost of the accelerometer. Electrostatically driven vibrating beam accelerometers, on the other hand, require a near perfect vacuum for proper operation. More specifically, squeeze film damping resulting from the vibratory action of the vibrating beams lowers the mechanical Q of the accelerometer. In order to optimize the mechanical Q, the vibrating beams are subjected to a vacuum to reduce the squeeze film damping. However, the housing for maintaining such a vacuum normally requires a leak rate on the order of 10.sup.-12 cc/sec. Such a housing is relatively expensive.